1. Field of the Invention
The present invention relates to a developing device in an image forming apparatus, which supplies electrically-charged toners by a supplying roller to a developing roller and then forms an image by means of an electro-photography by using the toners.
2. Description of the Related Art
There is an image forming apparatus, such as a printer or the like, which forms and records an image onto a record paper by means of a so-called electro-photography by use of a developing device, which is provided with at least a photosensitive drum servicing as a photosensitive body and a developing roller.
In this image forming apparatus, when toners are supplied to the developing roller, the toners are firstly adhered to an outer circumference of a supplying roller. Then, the developing roller and the supplying roller are mutually rotated in the same directions (for example, both are clockwise rotated) under the condition that the supplying roller to which the toners are adhered is pushed against the developing roller, and thereby the toners adhered to the supplying roller are supplied to the developing roller. After that, the toners supplied to the developing roller are adhered to an electric-static latent image on the photosensitive drum, and accordingly the development is performed.
In the developing device in the above mentioned image forming apparatus, the development is performed by using, for example, a toner made of a non-magnet material of a single component. However, in this developing method, the surface of the outer circumference of the supplying roller is made of a sponge material having a predetermined hardness (for example, in a case of an urethane foam roller, a hardness in which a cell number is equal to or more than 80/inch and an ASKER-C hardness ranges between approximate 30 and 20). The supplying roller is deformed so as to be collapsed in a radius direction thereof by pushing the supplying roller against the developing roller made of an elastic member such as rubber or the like. Then, the supplying roller is made in contact with the developing roller at a predetermined area, and accordingly the toners are supplied. Here, the deformation amount in the radius direction of the supplying roller is tentatively referred to as a collapse amount.
The reasons why the toners are supplied by rotating both of the supplying roller and the developing roller in the same directions under the condition that the supplying roller is pushed against the developing roller are described below. That is, as a first reason, when both are rotated in the same directions, the supplying roller and the developing roller are in contact with each other while they are moving in the directions opposite to each other, at the portion (i.e. the nip portion) where they are in contact with each other. However, it is intended to use the friction at that time to thereby charge the toners, which is a first function of the supplying roller. As a second reason, in order to prevent an image in a previous development from having an influence onto a subsequent development, it is intended to scrape off the toners, which are not electrically adhered from the developing roller onto the photosensitive drum in the previous development and thereby remains on the surface of the developing roller, which is a second function of the supplying roller. And as a third reason, it is intended to substantially uniformly supply the toners to the developing roller, which is a third function of the supplying roller.
At this time, in the developing device of the above mentioned image forming apparatus, the collapse amount is typically set to a value less than 0.5 mm. Moreover, a rotational speed of the supplying roller is set such that, in order to sufficiently exhibit the second function of the supplying roller especially among the above mentioned three functions of the supplying roller, the result of dividing an absolute value of a line speed on the outermost circumference of the supplying roller by an absolute value of a line speed on the outermost circumference of the developing roller is approximate 0.7.
Recently, it is extremely desired to make a printing speed faster. However, for this end, it is also necessary to make the developing speed in the developing roller and the photosensitive drum faster. Thus, it is necessary to increase a rotational speed of the developing roller. Hence, it is also necessary to make the rotational speed of the supplying roller faster to thereby maintain the ratio between the above mentioned speeds (approximate 0.7).
However, for example, if increasing the line speed on the outermost circumference in the developing roller equal to or more than 70 mm/second, the abrasion of the developing roller becomes severe because of the friction with the supplying roller. This results in the problem that a life of the developing roller becomes shorter and also a life of the developing device itself becomes short.